1. Field of the Invention
The invention relates to a method of measuring the quality of radio-transmission channels of a radio-transmission system.
The received field strength is often used to measure the quality of radio-transmission channels, more specifically for registering interference within the useful information.
Such a quality measuring method as the disadvantage that a measure of the interferences produced can only then be obtained when the power of the HF-noise signal can be considered as being constant. However, in a radio-transmission system this assumption is usually not valid. On the one hand the noise power of the receiver input noise changes in response to temperature fluctuations and on the other hand external noise sources such as, for example, co-channel transmitters (such as they can be present in a cellular radio-transmission system) cause a varying HF noise power. These noise powers which significantly fluctuate in mobile radio stations result in conditions such that the field strength is not suitable for use as a quality indicator, particularly in interference-limited cellular radio networks.
2. Description of the Prior Art
DE-AS No. 27 38 800 discloses a radio message transmission system with frequency and/or phase modulation, in which the amplitude fluctuations caused by interferences in the RF-sub-carrier signal are used for quantitively registering the interference. A comparatively constant power of the received useful signal is a prerequisite. In radio transmission systems including mobile radio stations, amplitude fluctuations occur however in response to fading and radio masking, which are present with the same order of magnitude both in good receiving conditions and also in poor receiving conditions, so that measuring the amplitude fluctuations of the RF-carrier signal is not suitable for use as a quality indicator.
From Jakes, Microwave Mobile Communications, John Wiley, 1974, it is known to determine the signal-to-noise ratio in the base band after demodulation. In this situation the noise outside the useful band is measured (out-of-band noise), this measurement often not being suitable for use in bandwidth-efficient digital modulation methods as the band of the low-frequency useful signals is not or is hardly limited. The signal-to-noise ratio in the baseband after the demodulation is often also not ascertainable in a digital radio transmission system.
In addition, DE-AS No. 26 12 476 discloses a radio telephone system in which a signal-to-noise interval measurement is effected, using a test tone. In this disclosure the LF-signal-to-noise ratio is measured with the aid of the pilot tone (test tone) located above the useful frequency band, no signal being transmitted during the transmission of the pilot tone in the useful LF-band, so that the noise power can be measured.
This method of quality measurement has however the disadvantage that during a message transmission in progress no noise measurements can be effected.
Additionally, IEEE Journal of Selected Areas in Communications, Vol. SAC-2, No. 4, July 1984, "Digital Portable Transceiver Using GMSK Modems and ADM Codec," H. Suzuki et al, describes a method of quality measurement in which the jitter of the received RF carrier is used for noise measurement.
However, this method can only be used in coherent demodulation methods in which the carrier is tracked phase-synchronously in the receiver. Modulation methods of this type are however not yet suitable for use in radio transmission systems comprising fast moving radio stations, and with higher carrying frequencies.
In addition, in the digital transmission of the message the jitter of the received and recovered binary signal is often used as a measure of the quality.
Such a method of quality measurement has however the disadvantage that in contemporary digital radio transmission systems with bandwidth-efficient modulation methods such as TFM or GMSK, the clock jitter alone is difficult to access. Especially in rapidly moving receivers a basic jitter is already produced by "random FM". Particularly at higher data rates the jitter is therefore not suitable for judging the receiving quality.
The invention has for its object to measure in a simple way and manner the quality of the radio-transmission channel during the transmission in progress, in a frequency or/and phase modulated-carrier radio transmission system. During the quality measurement also the value of co-channel transmission interferences, such as they may occur in a cellular radio transmission system, must be determined and the method of measuring the quality must be independent of the modulation method used in the radio transmission system.